I know you've heard it before, but an object is worth what somebody is willing to pay for it! So, one of the few sources of what a pin is worth is someplace like ebay. Right now, there is a Ying-Yang Mickey Head 'error' pin on ebay with a minimum bid of $10, and nobody has bid on it. And none have sold in the last month, which tells me they are worth less than $10. Pins like this one were not limited and were probably produced in the tens of thousands. And there is just not enough demand for a pin like this to make it 'worth' anything monetarily. But sentimentally, that may be a different story!